Resource reservation protocol—traffic engineering (RSVP-TE) uses an algorithm, for example, the constrained shortest-path first (CSPF) algorithm, to route (or signal) tunnels (e.g., label switch paths [LSPs]) within or through a network. CSPF requires that that a path be found through the network with sufficient reservable bandwidth along the entire path length in order to route a tunnel.
If it is not possible to find any path through the network through interfaces with enough available bandwidth for a tunnel, the tunnel will not be routed. Traffic that would use that tunnel to traverse the network needs to find another path, which in the simplest case will be the Internet protocol (IP) shortest path across the network. This path will be used irrespective of traffic on the path or resulting congestion. Furthermore, this path is typically the most congested.
This all-or-nothing approach leads to several problems. As a first example problem, traffic that would have routed over the tunnel now, typically, routes over the shortest interior gateway protocol (IGP) path, without regard for reservable bandwidth or current congestion conditions, which exacerbates the bandwidth shortage. Furthermore, the shortest IGP path is typically also the most congested in a network. As another example problem, other path constraints on the tunnel besides the bandwidth constraint, such as explicit hops, quality of service (QoS), and/or affinities settings, are ignored when the traffic goes unrouted. As a third example problem, the auto-bandwidth functionality, which is commonly used in some network systems, is unable to estimate future setup bandwidth values to request for the tunnel when the traffic goes unrouted. As such, with this example problem, if the tunnel is down or does not exist in the first instance, the auto-bandwidth measurements cannot be updated correctly (or at all).
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